1. Field of the Invention
This invention relates to an optical recording medium, and more particularly, to an optical recording disk of the write-once type compatible with the compact disk and a method for recording and reproducing the medium.
2. Prior Art
Japanese Patent Publication (JP-B) No. 75943/1991 discloses an optical information recording medium capable of reproducing signals in accordance with the compact disk (often referred to as CD) standard. In the examples of this publication, an optical recording disk is fabricated by forming a light absorption layer containing an organic dye on a light transmissive resin substrate, forming a light reflection layer thereon with or without an intervening hard layer, and forming a resinous protective layer thereon. Signals are recorded in the optical recording disk by directing a laser beam of 6 mW to the light absorption layer at a relative linear velocity of 1.2 m/s and reproduction signals are read out in accordance with the CD standard. At spots where the laser beam is irradiated, the light transmissive resin substrate at the surface is deformed in a convex fashion to form pits which accompany a phase difference to enable reading of the recorded information.
A similar write-once type optical disk is described in Nikkei Electronics, Jan. 23, 1989, No. 465, page 107. This optical disk has on a polycarbonate substrate, a recording layer of organic dye, a reflective layer of gold, and a protective layer in the described order. Upon irradiation of a recording laser beam, the organic dye layer absorbs light and is melted or decomposed and the substrate is softened at the same time whereupon the dye and substrate components are mixed at the interface so that the interface is eventually deformed to form record pits. The thus formed pits afford low reflectivity due to an optical phase difference as found in the CDs.
As opposed to the advantage of CDs that since pits are formed at the same time as injection molding of substrates and are thus sufficiently precise in shape and size to minimize jitter, the above-mentioned optical recording disks often suffer from increased jitter because pits are formed by deformation of the substrate.
The optical recording disk of JP-B 75943/1991 has a further problem that recording requires the same length of time as reproduction because the linear velocity during recording is the same as that during reproduction.